Why Can't We 'Monger' French Fries?

"Why is it that the only thing you can monger are fear and fish?" wonders Stephen Colbert on Twitter. Deep. Well, for starters, you can also monger war and gossip. In fact, you can theoretically monger a great deal of things. The closest synonym to "monger" is probably "peddle," both verbs having a slight negative connotation. Colloquially, though, we only pair "monger" with a few other words. Why is that?

And then there's another question: given that "monger" does have that negative connotation, are English speakers really as down on fish as they are on war, gossip, and fear?